Properties and Reactions of Chromium: A Chemical Analysis

This report provides a comprehensive overview of chromium, covering its history, properties, and chemical behavior. It details the discovery and historical uses of chromium, followed by a discussion of its physical and chemical properties, including its electronic configuration, oxidation states, and reactivity. The report delves into the chemistry of chromium, explaining concepts such as electrolytic reduction and crystal field theory, and how these principles influence the formation and behavior of chromium compounds. Various coordination complexes are explored, along with the role of pH in influencing the formation of different chromium species. Furthermore, the report highlights the diverse applications of chromium in metallurgy (stainless steel, electroplating), wood preservation, dyes and pigments, refractory materials, tanning, and catalysis. The report also includes detailed examples of chromium reactions, illustrating its behavior in different chemical environments and its interactions with other substances, and showing the chemical equations and reaction mechanisms, and the chemical structure of complexes of chromium.

Properties and reactions of Chromium
History of chromium
During 259 – 210 B.C.E, the Chinese belonging to the Qin dynasty used chromium (III)
oxide for coating various weapons. They coated swords and crossbow bolts by steel and
bronze which contained chromium. These weapons were excavated from the Terracotta Army
buried with the Emperor of China.
A German geologist and mineralogist called Johann Gottlob Lehmann discovered a mineral
which is orange red in colour in the Beryozovskoye mines in 1761. The mines were situated
in the Ural Mountains. He identified the compound as lead containing huge amounts of iron
and selenium and named it as Siberian red lead.
A French chemist and pharmacist named Nicolas-Louis Vauguelin argued that Siberian red
lead was a new elemental ore which he named as crocoite. (Figure 1) Pure chromium was
obtained from mineral containing chromium called crocoite for the first time by him in the
year 1797. He allowed crocoites to react with potassium carbonate to form chromic acid
which was reduced with carbon to form chromium in a graphite crucible. The name of the
element was obtained from the Greek word chroma which means colour. Another uncommon
Cr2O3 mineral is Eskolaite which was discovered in eastern Finland in Outokumpu ore and
named after Pentti Eskola, a geologist from Finland.
The principal commercial ore of chromium is a dark mineral called chromite which was
discovered by Lowitz and Klaporth separately in Beres of Mines. Chromite gets crystallised
from magma during the process of cooling, which is formed in deep ultramafic magmas. It is
present in the serpentines which is a form of metamorphic rocks.

Crocoite (Lunk, 2015)
Occurrence
Chromium ranks as the seventeenth element to be found abundantly in the earth’s mantle.
(Avudainayagam et al. , 2003) It is present in the nature as and tarapacaite (K2CrO4), chromite
(FeCr2O4) crocoite (PbCrO4), vauquelinite (CuPb2CrO4PO4OH) and bentorite
[Ca6(Cr,Al)2(SO4)3]. (Babula et al., 2008) Chromium exists in variable oxidation state: the
hexavalent state Cr (VI), the trivalent state Cr (III) and Cr (0) respectively. Cr (VI) is found
mainly in the chromate and dichromate (CrO42-) form. It is the most toxic stae of chromium
based on its high solubility, oxidation potential and mobility through the biological
membranes. The Cr (III) state is relatively less toxic due to its low mobility, less solubility is
water and its tendency to form strong bond with the organic matters in soil. However Cr (III)
gets oxidised to Cr (VI) in presence of managanese oxides and oxygen.(Peralta-Videaet al.,
2009) The metallic form of Chromium is referred as Cr (0), which is manufactured in
industries and used mainly for making alloys.
Properties
Chromium is a transition metal, which belongs to the class of refractory metal, having a
melting point more than that of platinum. It is very reactive in air and forms a layer of oxide
over its surface when exposed to air for a long duration. It is silver grey in colour and can be
polished to attain a smooth surface. It forms green chromic oxide when heated in air. There

are three main isotopes of chromium -52Cr, 53Cr and 54Cr respectively. Among the three
isotopes, the abundance of 52Cr is about 83.8%. There are 19 radioisotopes of chromium, 17
of which have a half life less than 24 hours to 1 min. However the radioisotopes 50Cr and
51Cr with a half life of 1.8 x 1017 years and 27.7 days respectively are substantially stable.
A deep insight into the properties of chromium can be obtained by having a knowledge about
the change of the metal from one oxidation state to another. The oxidation state Cr (III) is the
most common oxidation state of chromium metal. The reduction potential gives the energy
required for changing the oxidation state of the metal. The properties of chromium are shown
in Table 1.
Atomic number 24 Electronic configuration 1s22s22p63s23p63d54s1
Molar mass 51.9961 gmol-1 Electronegativity 1.66
Group in periodic table 6 Ionisation energy 652.9 KJmol-1
Period in periodic table 4 Atomic radius 128 pm
Crystal structure Body centered
cubic
Covalent radius 139
Cell parameter 291 pm Melting point 1907oC
Density 7.19 g cm-3 Boiling point 2671oC
Table 1: Physical properties of chromium metal (Lunk, 2015)
Chemistry of chromium
Purification of chromium by electrolytic reduction
The electric cell consists of an anode made of lead and cathode made of stainless steel. The
mixing of the chromic acid and sulphuric acid formed at the anode with the electrolyte at the
cathode is prevented by placing a diaphragm in between the two compartments. The
commercial electrolytic reduction process is carried out with potassium chromium(III) sulfate

dodecahydrate, KCr(SO4)2·12H2O. The complex is dissolved in hot water and placed in the
cathode of the electrolytic cell. On passing electricity the chromium gets deposited on the
stainless steel plate at cathode from the impure metal complex. The chromium is scrapped
from the cathode after 72 hours and heated in stainless steel cans at 420oC. This process
removes the hydrogen and water from the extracted chromium. The oxygen present in this
chromium is further removed by heating to 1400oC at 13 Pa pressure which furnishes
chromium metal with 99.9% purity. However this process of producing pure chromium is
becoming less popular due to the strict rules which has to be abided taking environmental
pollution as concern.
Crystal field theory (CFT)
The interaction between the surrounding ligands and the metal is explained by CFT. The
negatively charged ligands exhibit anion-cation interaction while the neutral ligands show
cation –dipole interaction with the metal atom (Boeyens, 2005). The ligands are responsible
for the energy levels of the central metal ion. The electronic configuration of Cr (III) is as
follows:
Cr3+ = [Ar] 3d3
The five degenerate (dxy, dyz , dzx , dx2-y2 , dz2) orbitals will split into two non degenerate
levels. The orbitals t2g (dxy, dyz , dzx) will have lower energy while eg (dx2-y2 , dz2) will have
higher energy. (Figure 2) The three electrons will occupy the lowest orbitals
according to Hund’s rule in the ground state.

Figure 2: The CFT splitting of Cr(III) in octahedral complex(Lunk, 2015)
An electron moves from the t2g orbital to eg orbital when a photon hits Cr (III) and absorbs
light in the visible spectrum. Thus the compound appears to be violet in colour in solution.
Co-ordination of Cr (III)
Cr (III) forms [Cr (OH2)6]3+ with water, which an octahedral metal complexes. The aqua
complexes mostly have co-ordination number six. These complexes in solution are acidic in
nature with pKa value of about 4.3 due to the release of proton from the water ligands.
[ Cr ( OH2 )6 ]3 +¿+H 2 O → [Cr (OH 2 )5 (OH )]2+ ¿+ H3 O+¿ ¿ ¿
¿
Role of pH
The pH plays a significant role in the case of chromium aqua complexes. The [Cr (OH2)6]3+
ion is predominant at very low pH and exhibits light blue colour. With the increase in pH,
the solution turns dark blue due to the presence of [Cr (OH)n (OH2)6-n](3-n)+ where n =1,2. A
further increase of pH gives rise to grey blue precipitate due to the formation of amphoteric
chromium (III) hydroxide Cr (OH)3 while at a very high pH, green tetrahydrooxochromate
(III) anion [Cr (OH)4]- is obtained. (Figure 3)

Figure 3: Schematic representation of the pH range of chromium complex. (Tytko, 1979)
Preparation of [Cr (OH2)6]3+ complex
It is a challenge to synthesize pure [Cr (OH2)6]3+ complex as one or more water molecules
are generally replaced by other ligands like Cl-. The substitution of chloride ion by water
molecule is not possible as the rate of exchange of ligands for the complex is very slow. A
convenient method for preparation of the complex is to reduce chromic acid in presence of
perchloric acid by hydrogen peroxide. (Marczak & Wrona, 1988)
Polyoxotunstate compounds of Cr (III)
The polyoxometalates occupy a significant place in inorganic chemistry based on its practical
applications. (Huheey et al., 2014)
Co-ordination of Cr (VI)
Role of pH

The Cr (VI) co-ordinates with only oxide ions at very high pH , but it cannot form bonds
with six oxide ions due to steric hindrance. Thus Cr (VI) form bonds with 4 monochromate
anion [(CrO4)2-] and a complex of yellow colour is formed. The monochromate anion is
protonated at lower pH to furnish hydrogen monochromate anion [(O3CrOH)-] , which
dimerises later to form dichromate anion as shown in the following equation:
Cr O4
2−¿+ H+ ¿→ [ O3 CrOH ] −¿ ¿
¿ ¿
2 [ O3 CrOH ]−¿→ Cr2 O7
2−¿+ H 2O ¿¿
The dichromate anion is formed because the monomer gets very acidic due to the presence of
the hexavalent chromium which induces the process of dimerisation. The equilibrium
between the monomer and the dimer of chromate anioin depends on the factors like
concentration of chromium and pH. Thus the monochromate anion is favoured in alkaline
condition while dichromate anion is favoured in acidic solution. (Scheme 1)
Scheme 1: The structure of monochromate and dichromate anions
Applications of chromium
Metallurgy
The two major use of chromium in metallurgy is electroplating and to make stainless steel
corrosion resistance and resistance to colour change by adding chromium to the alloy.

Chromising and electroplating are two techniques for creating a layer of chromium on the
surface of different metals. The elecroplating for decoration is usually done on nickel surface
by using chromic acid. (Figure 4) Chromium electroplating is preferred due to its lower
coefficient of friction. Chromium carbide (Cr3C2) obtained by reaction of chromium oxide
with carbon black, behaves as an inhibitor to the grain growth during the manufacture of WC-
Co hardmetals.(Pierson, 1996) It is also used during the synthesis of coatings used for
thermal spraying. (Zackrisson et al., 1998)
3 Cr2 O3 +13 C →2 Cr3 C2+ 9 CO
Figure 4: Decorative chrome plating(Lunk, 2015)
Preservative for wood
The salts obtained from Cr (VI) are very toxic and thus used for the wood preservation.
Timber is treated with chromated copper arsenate for the prevention of the decay of wood
from termite attack, marine borers and fungi. The lignin and cellulose is fixed with the aid of
chromium.
Dyes and pigments
Chrome yellow obtained from lead monochromate (PbCrO4) is a common dye used for
colouring various objects. It is synthesized by mixing potassium chromate with lead nitrate

when the precipitate of PbCrO4 is obtained. The main asset of chrome yellow is that it is
resistant to degradation, but have a tendency to become dark due to the formation of Cr2O3.
Synthesis of refractories
The compounds Cr2O3 and FeCr2O4has been widely used as refractory materials like cement
kilns, foundry sands and blast furnaces based on its high melting points and hest resistivity.
The refractories are synthesised from magnesite and chromite.
Tanning
Chromium alum [KCr (SO4)2.12H2O] is a chromium (III) salt which is used for leather
tanning. The collagen fibres are cross- linked by Cr (III) which stabilises the leather. The
leather tanned with chromium generally consists of 4-5% of chromium which is strongly
bound to the proteins. The tanning floats are reused in the chrome tanning process. However
the reused number of the tanning floats depends on the quality of the leather. (Morrera et al.,
2011)
Catalyst
Polyethylene is synthesized by the use of Phillips catalyst which is made by depositing CrO3
on silica. Chromium complexes are used as catalysts for the preparation of various
hydrocarbons. The mixed oxides of Fe-Cr are used as catalysts at very high temperature in
the water shift gas reaction. Copper chromite (Cu2Cr2O5) is used as a catalyst for
hydrogenation.
Reactions of chromium
The Cr (III) ions react with NaOH or alkaline NH3 to form precipitate of Cr (III) hydroxide
which is green in colour.
[ Cr ( H2 O )6 ]
3 +¿ ( aq ) + 3O H −¿→ [Cr ( OH )3 ( H 2O ) 3 ]( s ) + 3H 2O (l) ¿
¿

The Cr (III) complex has an octahedral geometry which reacts with acid to form
salts as shown below:
[ Cr ( OH ) 3 ( H2 O ) 3 ] ( s ) +3 H3 O+ ¿ ( aq ) → [ Cr ( H 2 O )6 ] 3+ ¿ ( aq ) + 3H 2O (l) ¿
¿
Cr (III) hydroxide exhibits amphoteric nature since the Cr (III) hydroxide gets
easily dissolved in excess amount of strong alkalis to produce a dark green
solution. The complex does not become soluble in weak bases like dilute Na2CO3
solution.
[Cr (H2O)6]3+ complex
The octahedral aqueous complex of Cr (III) hydroxide reacts with excess if chloride ion to
form tetrachlorochromate (III) ion as shown below:
[ Cr ( H2 O )6 ]3 +¿ ( aq )+ 4 Cl−¿(aq)→ [Cr (Cl )4 ]
−¿ ( aq )+ 6H 2O( l)¿
¿ ¿

Tetrachlorochromate (III) ion
In the similar lines, the octahedral aqueous complex of Cr (III) hydroxide and Cr (II)
hydroxide reacts with ethylene diamine tetraacetate (EDTA) to form Cr (III) complexes with
EDTA ion as shown below:
[ Cr ( H2 O )6 ]3 +¿ ( aq )+ EDTA4−¿(aq) → [Cr (EDTA ) ]−¿ ( aq ) +6H 2O (l) ¿
¿¿
[ Cr ( H2 O )6 ]2+¿ ( aq )+ EDTA4−¿(aq)→ [Cr (EDTA ) ]2−¿ ( aq )+ 6H 2O (l)¿
¿¿
The reaction of Cr (III) complexes with H2O2 in presence of alkali produces a yellow solution
due to the formation of chromate (VI) ion as shown below:
2 Cr ( H2 O )3 ( s )+3 H2 O2 ( aq ) +4 OH−¿(aq)→ [2 CrO 4 ]2−¿ ( aq )+ 8 H2 O(l) ¿
¿
The chromate (VI) ion on treatment with acid furnishes dichromate (VI) ions with orange
colour, as shown below:
[ 2 Cr O4 ]
2−¿ ( aq ) +2 H + ¿(aq)→ [ Cr2 O7 ] 2−¿( aq ) +H 2O (l) ¿
¿¿
The chromate (VI) ion on reaction with Pb (II) ions gives Pb (II) chromate (VI) as precipitate,
which is yellow in colour as shown below:
[ Cr O4 ]2−¿ (aq )+ Pb2+¿( aq)→ PbCr O4 (s ) ¿¿
Similarly, the chromate (VI) ion on reaction with Ag (I) ions gives Ag chromate (VI) as
precipitate, which is dark red in colour as shown below:
[ Cr O4 ] 2−¿ ( aq ) +2 Ag+¿( aq)→ Ag2 Cr O4 ( s )¿ ¿
The potassium dichromate (VI) solution in presence of acid is used to oxidise excess organic
alcohols to aldehydes as shown below:

[ Cr2 O7 ] 2−¿ ( aq ) + 8 H + ¿ ( aq )+3 C H 3C H 2OH → 3C H 3CHO +7 H 2O ( l ) + 2Cr3+ ¿¿ ¿¿
When the amount of potassium dichromate (VI) solution is in excess, in presence of acid, it is
used to oxidise organic alcohols to acids as shown below:
2 [ Cr2 O7 ]2−¿ (aq )+16 H +¿ ( aq )+ 3C H 3C H 2OH →3 C H3 COOH +11 H 2O (l ) +4 Cr3+¿ ¿ ¿¿
Cr(NO3)3.9H2O reacts with 2,6-diactylpyridinedisemicarbazone (DAPSC), to generate two
different kinds of crystal complexes. At zero pH the protonated ligand [Cr(DAPSC)(H2O)2]
(NO3)3.2H2O is obtained while at pH = 3 the complex [Cr(DAPSC-H)(H2O)2](NO3)2.H2O is
produced. (Bino et al., 1987)
Cr (III) reacts with p-vanillin semicabazone and vanillinthiosemicabazoneto form the
octahedral complex shown in Figure 5, which exhibited antimicrobial activities.
Figure 5: Octahedral complex of Cr (III) (Chandra & Tyagi, 2008)
The reaction of 4-hydroxycoumerine-3-carbaldehyde with thiocabohydrazide in the molar
ratio 2:1 formed a Schiff’s base called bis-[4- hydroxycoumerin-3-yl]-1N,5N-

thiocarbohydrazone which acts as a ligand and forms complex with Cr (III) as shown in
figure 6
Figure 6: Complex of Cr (III) (Abou-Melha et al., 2008)
Another Schiff’s base was synthesized from o-phenylenediamine and salicylaldehyde and
was allowed to form a complex with Cr (III). Figure 7
Figure 7: Octahedral complex of Cr (III) (El-Ajaily et al., 2007)
Conclusion
Chromium is a transition metal in the first row of the periodic table, which occupies a
significant place in chemistry due to its industrial and analytical applications. Metallic
chromium is a primary requirement in steel industries as chromium mixed steel possesses
strength and hardness. It has the ability to form different complexes, of which Cr (III) and Cr

(VI) complexes are more stable. Among the two, the complexes formed with Cr (III) are
more common as special interests is being focussed on the mixed ligand Cr (III) complexes
along with dichromium (III) compounds. The metal and its complexes have a wide variety of
uses in different areas like tanning industries, metallurgy, research and chrome plating. Thus
there is still an investigation for synthesizing new chromium complexes with wider
applications.

References
Abou-Melha, K. S. & H. Faruk, H. (2008). Journal of Iranian Chemical Society, 5, 122.
Avudainayagam, S., Megharaj, M., Owens, G., Kookana, R. S., Chittleborough, D. & Naidu,
R. (2003). Chemistry of chromium in soils with emphasis on tannery waste sites, Reviews of
Environmental Contamination and Toxicology, 178, 53–91.
Babula, P., Adam, V., Opatrilova, R., Zehnalek, J., Havel, L. & Kizek, R. (2008).
Uncommon heavy metals, metalloids and their plant toxicity: a review. Environmental
Chemistry Letters, 6 (4), 189–213.
Bino, A., Frim, R. & Genderen, M. V. (1987) Inorganica Chimica Acta, 127, 95.
Boeyens, J.C.A. (2005) New theories for chemistry. Elsevier, Amsterdam.
Chandra, S & Tyagi, M. (2008). Journal of Indian Chemical Society, 85, 42.
El-Ajaily, M. M. , Abdlseed, F. A. & Ben-Gweirif, S. (2007), e-Journal of Chemistry, 4 ,
461.
Huheey, J. E., Keiter, E. A. & Keiter, R.L. (2014) Anorganische Chemie —Prinzipien von
Struktur und Reaktivita¨t. Walter de Gruyter GmbH, Berlin/Boston, pp 967–984.
Lunk, H. J. (2015). Discovery, properties and applications of chromium and its compounds.
Chem Texts , 1(6) , 3-17.
Marczak, S. & Wrona, P.K. (1988) Journal of Electroanalytical Chemistry, 247, 215–227.
Morera, J.M., Bartolı,´ E., Chico, R., Sole,´ C. & Cabeza, L.F. (2011) Online Version
Journal of Clean Production, 19, 2128–2132

Peralta-Videa, J. R., Lopez, M. L., Narayan, M., Saupe, G., & Gardea-Torresdey, J. (2009).
The biochemistry of environmental heavy metal uptake by plants: implications for the food
chain. International Journal of Biochemistry and Cell Biology, 41(8-9), 1665–1677.
Tytko, K. H. (1979). Chemie in unserer Zeit, 13, 184–194.
Pierson HO (1996) Handbook of refractory carbides and nitrides: properties, characteristics,
processing and applications. Noyes, Westwood, NJ.
Zackrisson, J., Jansson, B., Upadhyaya, G.S., Andre´n, H.O. (1998). International Journal of
Refractory Metal and Hard Materials, 16, 417–422.